The present disclosure relates to casting cores, and in particular to investment casting cores which are formed at least in part from refractory metals made by a tomo-lithographic manufacturing process.
Investment casting is a commonly used technique for forming metallic components having complex geometries, especially hollow components, and is used in the fabrication of superalloy gas turbine engine components (i.e. components comprised of materials such as, for example, single-crystal PWA1480, single-crystal PWA1484, columnar grain PWA1422, columnar grain PWA1426, and other nickel-base alloys). Gas turbine engines are widely used in aircraft propulsion, electric power generation, and ship propulsion. In all gas turbine engine applications, efficiency is a prime objective.
Improved gas turbine engine efficiency can be obtained by operating at higher temperatures, however current operating temperatures are at such a level that, in the turbine section, the superalloy materials used have limited mechanical properties. Consequently, it is a general practice to provide air cooling for components in the hottest portions of gas turbine engines, typically in the turbine section. Cooling is provided by flowing relatively cool air from the compressor section of the engine through passages in the turbine components to be cooled. It will be appreciated that cooling comes with an associated cost in engine efficiency, consequently, there is a strong desire to provide enhanced specific cooling, maximizing the amount of cooling benefit obtained from a given amount of cooling air.
Referring to FIG. 1, a gas turbine engine 10 includes a compressor 12, a combustor 14, and a turbine 16. Air 18 flows axially through the sections 12, 14, and 16 of the engine 10. As is well known in the art, air 18, compressed in the compressor 12, is mixed with fuel which is burned in the combustor 14 and expanded in the turbine 16, thereby rotating the turbine 16 and driving the compressor 12.
Both the compressor 12 and the turbine 16 are comprised of rotating and stationary airfoils 20, 22, respectively. The airfoils, especially those disposed in the turbine 16, are subjected to repetitive thermal cycling under widely ranging temperatures and pressures. To avoid thermal damage to the airfoils, each airfoil 20 includes internal cooling.
Referring to FIG. 2, the airfoil 20 includes a leading edge 26 and a trailing edge 28 extending from a root end 30 to a tip 32 thereof and a platform 34. A leading edge cooling passage 40 is formed within the leading edge 26 of the airfoil 20 having radially extending, connected channels 42-44 and a leading edge inlet 46, formed within the platform 34 and in fluid communication with the channel 42. A plurality of leading edge crossover holes 48 formed within a leading edge passage wall 50 separating the channel 44 from a leading edge exhaust passage 52, allow the cooling air from the channel 44 to flow into the leading edge exhaust passage 52. A trailing edge cooling passage 56 is formed within the trailing edge 28 of the airfoil 20 having radially extending connected channels 58-60 and a trailing edge inlet 62 formed within the platform 34 and in fluid communication with the channel 58. A first plurality of trailing edge crossover holes 66 is formed within a first trailing edge wall 68 and a second plurality of trailing edge crossover holes 72 is formed within a second trailing edge wall 74 to allow cooling air from channel 58 to flow through an intermediate passage 78 to a plurality of trailing edge slots 80.
A ceramic core 120, as depicted in FIGS. 3 and 4, is used in the manufacturing process of the airfoils 20 and defines the hollow cavities therein. A ceramic core leading edge 126 and a ceramic core trailing edge 128 correspond to the leading edge 26 and trailing edge 28 in the airfoil 20, respectively. A ceramic core root 130 and a tip 132 correspond to the airfoil root 30 and tip 32, respectively. Ceramic core passages 140, 156 with channels 142-144, 158-160, and inlets 146, 162 respectively, correspond to passages 40, 56 with channels 42-44, 58-60 and inlets 46, 62, of the airfoil, respectively. Passages 52 and 78 of the airfoil correspond to channels 152 and 178 in the ceramic core. Pluralities of fingers 148, 166, 172 in the core 120 correspond to the plurality of crossover holes 48, 66, 72 in the airfoil 20, respectively. A core tip 190 is attached to the core passages 140, 156 by means of fingers 182-185, to stabilize the core 120 at the tip 132. An external ceramic handle 194 is attached at the core trailing edge 128 for handling purposes. A core extension 196 defines a cooling passage at the root to the airfoil 20. Centerlines 197-199 extend radially through each row of fingers 148, 166, 172, respectively.
While turbine blades and vanes are some of the most important components that are cooled, other components such as combustion chambers and blade outer air seals also require cooling, and such components (and in fact all complex cast articles) can include similar structures to those shown in FIG. 2 and be made using ceramic cores with general similarities to those shown in FIGS. 3 and 4.
Conventionally, cores such as that shown in FIGS. 3 and 4 are fabricated from ceramic materials but such ceramic cores are fragile, especially the advanced cores used to fabricate small intricate cooling passages in advanced hardware. Current ceramic cores are prone to warpage and fracture during fabrication and during casting. In some advanced experimental blade designs casting yields of less than 10% are achieved, principally because of core failure.
Conventional ceramic cores are produced by a molding process using a ceramic slurry and a shaped die (not shown); both injection molding and transfer-molding techniques may be employed. The pattern material is most commonly wax although plastics, low melting-point metals, and organic compounds such as urea, have also been employed. The shell mold (not shown) is formed using a colloidal silica binder to bind together ceramic particles which may be alumina, silica, zirconia, and alumina silicates.
The investment casting process to produce a turbine blade, using a ceramic core, will be explained briefly here (although it is not depicted in any of the figures). A ceramic core having the geometry desired for the internal cooling passages is placed in a metal die whose walls surround but are generally spaced away from the core. The die is filled with a disposable pattern material such as wax. The die is removed leaving the ceramic core embedded in a wax pattern. The outer shell mold is then formed about the wax pattern by dipping the pattern in a ceramic slurry and then applying larger, dry ceramic particles to the slurry. This process is termed stuccoing. The stuccoed wax pattern, containing the core, is then dried and the stuccoing process repeated to provide the desired shell mold wall thickness. At this point the mold is thoroughly dried and heated to an elevated temperature to remove the wax material and strengthen the ceramic material.
The result is a ceramic mold containing a ceramic core which in combination define a mold cavity. It will be understood that the exterior of the core defines the passageway to be formed in the casting and the interior of the shell mold defines the external dimensions of the superalloy casting to be made. The core and shell may also define casting portions such as gates and risers which are necessary for the casting process but are not a part of the finished cast component.
After the removal of the wax, molten superalloy material is poured into the cavity defined by the shell mold and core assembly and solidified. The mold and core are than removed from the superalloy casting by a combination of mechanical and chemical means.